CN110467453B

CN110467453B - Method for producing ceramic molded body for sintering and method for producing ceramic sintered body

Info

Publication number: CN110467453B
Application number: CN201910386537.9A
Authority: CN
Inventors: 碇真宪; 松本卓士
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 2018-05-11
Filing date: 2019-05-10
Publication date: 2023-03-03
Anticipated expiration: 2039-05-10
Also published as: US11492294B2; EP3566842A1; CN110467453A; US20190345067A1; CN116041054A; EP3566842B1

Abstract

The present invention relates to a method for producing a ceramic compact for sintering and a method for producing a ceramic sintered body. A method of manufacturing a ceramic compact for sintering includes forming a raw material powder containing a ceramic powder and a thermoplastic resin having a glass transition temperature higher than room temperature into a predetermined shape by isostatic pressing, wherein a raw material powder slurry is prepared by adding the ceramic powder and the thermoplastic resin to a solvent so that the thermoplastic resin is present in an amount of 2 to 40 wt% of the total weight of the ceramic powder and the thermoplastic resin, a cast compact is formed by casting the raw material powder slurry into a predetermined shape, drying and first-stage isostatic pressing is applied at a temperature lower than the glass transition temperature of the thermoplastic resin to form a first-stage press compact, and then the first-stage press compact is heated to a temperature above the glass transition temperature of the thermoplastic resin, and Warm Isostatic Pressing (WIP) forming is performed as second-stage isostatic pressing to manufacture the ceramic compact.

Description

Method for producing ceramic molded body for sintering and method for producing ceramic sintered body

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims priority from patent application nos. 2018-092085 and 2019-073264 filed in japan, on 2018, month 11 and 2019, month 4, month 8, respectively, according to 35u.s.c. § 119 (a), the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates to a method of preparing a ceramic molded body (molded body) for sintering, in which residual voids are small and also the amount thereof is reduced by realizing pressure transfer and plastic flow at the time of molding, and a method of manufacturing a ceramic sintered body using the molded body manufactured by the manufacturing method.

Background

In general, it is preferable to reduce residual bubbles inside any sintered body of ceramics because of improvement in mechanical strength, thermal conductivity, optical transparency, electrical characteristics, long-term reliability, and the like. Heretofore, as a method for manufacturing thick ceramics with high yield, a method in which a powder press mold is molded has been widely adopted. As the most classical methods, there are a method in which Cold Isostatic Pressing (CIP) molding is performed after uniaxial pressing, and a method in which raw material powder is filled in a rubber mold and the like and directly subjected to CIP molding, and these methods have been widely adopted in industry up to now. Incidentally, in order to improve shape retention at the time of molding and prevent cracking at the time of molding and sintering, a thermoplastic resin (so-called binder) is often mixed in a ceramic powder starting material (raw material powder).

When the step of mixing and adding the thermoplastic resin to the raw material powder is adopted, it is possible to increase the crushing strength of the secondarily aggregated raw material powder and the granulated raw material in the case of granulation, so as to sufficiently transmit the pressure to the inside of the molded body at the time of press-molding the thick ceramic, thereby improving the molding density. Furthermore, it is possible to improve the shape retention of the molded body and thereby prevent cracking and denaturation in the subsequent step. In this way, a molded body having a relatively high molding density can be molded into a desired shape with high yield. However, on the other hand, there are also the following problems: plastic flow of the raw material powder and the raw material pellets at the time of uniaxial pressing and CIP molding is hindered, large internal residual stress is generated at the time of molding or bridging of the raw material powder, and intergranular voids are induced. Therefore, it is known that residual stress and residual bubbles exist inside, and thus various properties are reduced in the case of densifying the compact by sintering or the like.

Therefore, as a method of improving the plastic fluidity of the thermoplastic resin mixed with and added to the ceramic powder raw material and thereby promoting densification at the time of ceramic molding, warm Isostatic Pressing (WIP) molding is proposed. For example, patent document 1 (JP 2858972) discloses a method of manufacturing a ceramic molded body, in which in the case where a ceramic powder is mixed with a thermoplastic resin, the temperature is raised to a temperature region where the thermoplastic resin is thermally softened at isostatic pressing, the mixture is subjected to primary molding, rubber film formation, and secondary isostatic pressing, or the mixture is directly placed in a rubber mold or the like and subjected to isostatic pressing. Further, an isostatic pressing apparatus capable of raising and adjusting the temperature to a temperature region in which the thermoplastic resin is heat-softened is called "warm isostatic pressing (w.i.p)".

Incidentally, a warm isostatic pressing apparatus for uniformly heating the entire powder at a low temperature, so-called WIP apparatus itself, has been known for a long time and can be confirmed, for example, in a publicly known document such as patent document 2 (JP-B S54-14352). Further, an external circulation heating type WIP apparatus improved to have a more practical structure is also disclosed in patent document 3 (JP-a S61-124503).

The molding technique of ceramics mixed with thermoplastic resins using such WIP equipment was later used as a technique for a pressure bonding step of laminating ceramics. For example, patent document 4 (WO 2012/060402) discloses an example in which Warm Isostatic Pressing (WIP) can be employed and a known example in which thermocompression bonding is performed at a temperature of 80 ℃ and a pressure of 1 ton as a method of molding a laminated green sheet of an all-solid battery. Alternatively, patent document 5 (JP-a 2014-57021) discloses an example of performing Warm Isostatic Pressing (WIP) and exemplifies a form in which a laminated sheet is preheated to a predetermined temperature in a vacuum-packed state and then warm isostatic pressing is applied at a temperature of 70 ℃ as a press-forming method for press-forming a laminated ceramic electronic component such as a laminated ceramic capacitor.

However, when a thermoplastic resin is pressed in a heated state at a glass transition temperature or higher, there are generally problems as follows: plastic deformation and plastic flow mainly occur, and the force for transmitting the applied pressure to the inside of the molded body is extremely weakened. Therefore, even though the raw material powder whose shape retention and crush strength are enhanced and pressure transmission property is improved by specially mixing and adding the thermoplastic resin, there is a fatal defect that the pressure transmission to the inside of the molded body is rapidly attenuated and especially a large number of voids remain inside the thick molded body by performing the WIP process.

Therefore, although the WIP apparatus itself has been known for a long time, the WIP apparatus is used only when a (thin) sheet molded body having a thin thickness as described above is manufactured, and there is almost no example of using the WIP as a thick ceramic molding technique since the above-mentioned patent document 1.

Further, a cast molding method and an extrusion molding method are also known, in which a ceramic raw material is wet-kneaded with a thermoplastic resin and formed into a slurry, and the slurry is molded while being wet, in addition to a dry raw material. The wet shaped bodies formed by these shaping methods tend to be quite advantageous shaped bodies with less coarse and coarse voids after this step. It was judged that this is sufficient, and no known example of further subjecting these molded bodies to post-press molding has been found.

However, a ceramic sintered body obtained by a conventional method such as the cast molding method described above or the like is required to further improve various properties (such as optical transparency).

Incidentally, patent document 6 (JP 5523431) exemplifies an embodiment, but exemplifies a method of arbitrarily selecting any one of press forming of uniaxial press forming, cold Isostatic Pressing (CIP) of isostatic pressing, warm Isostatic Pressing (WIP), and Hot Isostatic Pressing (HIP), or isostatic pressing after uniaxial press forming as a molding raw material powder, and further exemplifies that the method may be die forming such as extrusion molding and cast molding, as another example. However, no specific description has been found in which the former molding step and the latter molding step are combined. Furthermore, no knowledge has been found in the prior art about how this method acts on the properties of the shaped body when the wet shaped body obtained by the cast molding method or the like is further subjected to subsequent CIP molding or WIP molding.

CITATION LIST

Patent document 1

Patent document 2

JP-A S61-124503 in patent document 3

Patent document 4

Patent document 5

Patent document 6

Summary of The Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a ceramic formed body for sintering, by which a dense ceramic sintered body having favorable various properties, whose residual voids are significantly small and which has no residual stress, can be produced, and a method for producing a ceramic sintered body, in which a ceramic formed body produced by the method for producing a ceramic formed body for sintering is used.

In order to achieve the above object, the present invention provides the following method of preparing a ceramic compact for sintering and the following method of manufacturing a ceramic sintered body.

1. A method for preparing a ceramic molded body for sintering, which is molded by isostatic pressing a raw material powder containing a ceramic powder and a thermoplastic resin having a glass transition temperature higher than room temperature into a predetermined shape, comprising the steps of:

preparing a raw material slip by adding a ceramic powder and a thermoplastic resin to a solvent so that the thermoplastic resin is present in an amount of 2 wt% or more and 40 wt% or less of the total weight of the ceramic powder and the thermoplastic resin;

molding a casting-molded body by casting the raw material powder slurry wet into a predetermined shape and drying;

molding a first-stage press-molded body by isostatic pressing the dried cast-molded body at a temperature lower than the glass transition temperature of the thermoplastic resin as a first-stage isostatic pressing; and

the ceramic compact is molded by Warm Isostatic Pressing (WIP) as a second-stage isostatic pressing with the body heated to a temperature equal to or higher than the glass transition temperature of the thermoplastic resin.

2. The method of claim 1, wherein the first stage isostatic pressing is Cold Isostatic Pressing (CIP) forming.

3. The method as set forth in claim 1, wherein after the first-stage press-formed body is formed, heating of the first-stage press-formed body is started while maintaining the first-stage isostatic state, and then WIP forming is performed as the second-stage isostatic pressing.

4. The process of any of claims 1 to 3, wherein the pressing medium in WIP forming is water or oil.

5. The method of any of claims 1 to 4, wherein the thermoplastic resin has a glass transition temperature above room temperature and below the boiling point of the pressing medium in WIP molding.

6. The method of any one of claims 1 to 5, wherein the thermoplastic resin is at least one selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, a copolymer of polyvinyl alcohol and polyvinyl acetate, methyl cellulose, ethyl cellulose, polyvinyl butyral, polyvinyl acrylate, and a copolymer of polyvinyl alcohol and polyvinyl acrylate.

7. The method of any one of claims 1 to 6, wherein the cast molding body is molded by centrifugal casting molding via solid-liquid separation of raw material slip.

8. A method of manufacturing a ceramic sintered body, which comprises the step of sintering a ceramic formed body produced by the method of producing a ceramic formed body for sintering described in any one of 1 to 7 in an inert atmosphere or in vacuum, and further Hot Isostatic Pressing (HIP) the sintered ceramic formed body.

9. The method of claim 8, further comprising the step of degreasing the ceramic shaped body prior to sintering.

10. The method as set forth in claim 8 or 9, further comprising the step of annealing the HIP-treated body after the HIP treatment.

Advantageous effects of the invention

According to the present invention, when a ceramic formed body, particularly a thick ceramic formed body, is press-formed, it is possible to effectively achieve pressure transmission to the inside of the formed body and plastic flow of a thermoplastic resin, and to manufacture a dense ceramic formed body in which residual voids are significantly small and residual stress is eliminated. Further, by sintering the ceramic formed body, a ceramic sintered body having a truly high density and few residual bubbles can be produced. As a result, a high-quality ceramic sintered body can be provided, which exhibits improved mechanical strength, thermal conductivity, optical transparency, and the like, and thus exhibits more advantageous properties than the prior art.

Description of the preferred embodiments

Method for producing ceramic shaped bodies for sintering

Hereinafter, the method of preparing a ceramic compact for sintering of the present invention is described. Incidentally, the room temperature herein is an ambient temperature in the step of forming the ceramic formed body for sintering, and is usually 25 ± 5 ℃.

A method of producing a ceramic compact for sintering of the present invention is a method of producing a ceramic compact for sintering, which is formed by isostatic pressing a raw material powder containing a ceramic powder and a thermoplastic resin having a glass transition temperature higher than room temperature into a predetermined shape, characterized by comprising the steps of: preparing a raw material slip by adding a ceramic powder and a thermoplastic resin to a solvent such that the thermoplastic resin is present in an amount of 2 wt% or more and 40 wt% or less of the total weight of the ceramic powder and the thermoplastic resin; molding a casting molding body by casting the raw material powder slurry wet into a predetermined shape and drying; molding a first-stage press-molded body by isostatic pressing the dried cast-molded body at a temperature lower than the glass transition temperature of the thermoplastic resin as a first-stage isostatic pressing; and molding a ceramic molded body by Warm Isostatic Pressing (WIP) as second-stage isostatic pressing with the body heated to a temperature equal to or higher than the glass transition temperature of the thermoplastic resin.

Hereinafter, the details of the present invention are described.

Raw material powder slurry

The raw material slip used in the present invention contains at least ceramic powder, a thermoplastic resin (binder), and a solvent as essential components, and is prepared by adding ceramic powder to the solvent so as to be contained in an amount of 60 wt% or more and 98 wt% or less and adding a thermoplastic resin (binder) having a glass transition temperature higher than room temperature to the solvent so as to be contained in an amount of 2 wt% or more and 40 wt% or less (based on the total weight of the ceramic powder and the thermoplastic resin). That is, the raw material slip is prepared by adding the ceramic powder and the thermoplastic resin to a solvent such that the thermoplastic resin is present in an amount of 2 wt% or more and 40 wt% or less of the total weight of the ceramic powder and the thermoplastic resin.

Wherein the ceramic powder constitutes a desired ceramic sintered body. The composition thereof is selected according to the intended properties, and is not particularly limited in the present invention. In other words, the ceramic powder may be an oxide or nitride or fluoride. Further, the present invention can be suitably employed even when the ceramic powder is a metal-based material such as an intermetallic compound.

For example, in the case of manufacturing a transparent ceramic sintered body for a faraday rotator, examples of preferred terbium-containing oxide materials to be selected may include the following three types: namely that

(i) A terbium-containing garnet-type oxide transparent ceramic composed of a sintered body of an oxide garnet (TAG-based composite oxide) containing Tb and Al as main components and Sc as another component,

(ii) A terbium-containing garnet-type oxide transparent ceramic having the composition formula Tb ₃ Ga ₅ O ₁₂ A sintered body composition of TGG composite oxide, and

(iii) A transparent terbium-containing bixbyite-type oxide ceramic represented by the following formula (A).

(Tb _x R _1-x ) ₂ O ₃ (A)

(in the formula (A), x is 0.4. Ltoreq. X.ltoreq.0.7, and R contains at least one element selected from scandium, yttrium, and lanthanoids other than terbium).

The material of (i) is further detailed.

(i) Is a terbium-containing oxide containing Tb and Al as main components and Sc as another component and having a garnet structure as a structure.

In the garnet structure, since the faraday rotation angle (Velde constant) per unit length becomes large, it is preferable that the composition ratio of terbium is high. In addition, since the crystal field of terbium has edges and the deformation of terbium ions is reduced, it is preferable that the composition ratio of aluminum is high. Further, since aluminum has the smallest ion radius among trivalent ions that can be stably present in the oxide of the garnet structure, whereby the lattice constant of the garnet structure can be lowered while the composition ratio of terbium ions remains as it is, whereby the faraday rotation angle (Velde constant) per unit length becomes large, the composition ratio of aluminum is preferably high. Further, since the thermal conductivity of the entire system is also improved, it is preferable that the composition ratio of aluminum in the garnet-type oxide is high.

Incidentally, when the cation sites of the entire system are occupied only by terbium and aluminum, the perovskite structure is further stabilized, which results in the formation of perovskite-type hetero phases. Here, scandium (Sc) is a material having an intermediate ionic radius, and thus may exist in terbium sites constituting a garnet structure and in some aluminum sites in the form of a solid solution. Scandium (Sc) is also a buffer material that can exist in solid solution form by adjusting its distribution ratio to terbium sites and aluminum sites so that the ratio of terbium to aluminum completely conforms to the stoichiometric ratio, and thereby minimizes the energy of crystallite generation in the case where the ratio of terbium to aluminum deviates from the stoichiometric ratio by a change in weighing. Therefore, a sintered body formed of a garnet composition single phase can be stably obtained, and scandium is thus a preferable added element.

Thus, for example, in the TAG basic composition formula (Tb) ₃ Al ₅ O ₁₂ ) Of these, it is preferable to replace a part of 0 or more and less than 0.08 in the total site amount 3 of terbium and a part of 0 or more and less than 0.16 in the total site amount 5 of aluminum with scandium (Sc) becauseFurther stabilizing the garnet structure.

In addition, some sites of terbium may be substituted by yttrium and lutetium. Neither yttrium nor lutetium is an obstacle because the ionic radius is smaller than terbium and further stabilizes the garnet structure. Further, even when terbium is substituted, yttrium and lutetium do not become obstacles because there is no absorption peak thereof in the oscillation wavelength band of 0.9 μm or more and 1.1 μm or less in the ordinary fiber laser system.

(iii) Tb by (ii) ₃ Ga ₅ O ₁₂ A garnet-type oxide material.

The material is a material having a garnet structure and composed of oxides of terbium (Tb) and gallium (Ga). In this structure, since the faraday rotation angle per unit length (Velde constant) is large, the composition ratio of terbium is preferably high. In addition, since the melting point is greatly lowered, the production temperature can be lowered, and the cost can be reduced, it is preferable that the composition ratio of gallium is high. In addition, tb ₃ Ga ₅ O ₁₂ Is preferred because it is widely used as a faraday rotator for fiber laser systems and accumulates its long term reliability data.

An oxide material having a bixbyite structure and represented by formula (a) of (iii) is described.

The material is an oxide material having a terbium oxide structure of the silsesquioxane type as a skeleton and a structure in which the site of the terbium ion, i.e., the terbium ion, is substituted with a large amount of at least one element selected from scandium, yttrium, and lanthanoids other than terbium in the range of 1-x (where 0.4. Ltoreq. X.ltoreq.0.7) in formula (A).

This structure is one in which the concentration of terbium ions in the sesquioxide structure is the highest among several terbium oxide structures. Therefore, since the faraday rotation angle (Velde constant) per unit length becomes large, the composition ratio of terbium is preferably high.

Furthermore, even when some of the terbium ions are substituted with other ions in the range of 1-x (where 0.4. Ltoreq. X.ltoreq.0.7) in the formula (A), the Faraday rotation angle per unit length (Velde constant) can be kept high, and therefore it is preferable that some of the terbium ions exhibiting absorption be substituted with other ions not exhibiting absorption, because the absorption density from the terbium ions per unit cell can be reduced.

In formula (A), x is preferably in the range of 0.4. Ltoreq. X.ltoreq.0.7 and still more preferably 0.4. Ltoreq. X.ltoreq.0.6. Since the faraday rotation angle per unit length (Velde constant) decreases, x less than 0.4 is not preferred. Further, since the absorption amount from terbium increases to a non-negligible level, x exceeding 0.7 is not preferable.

Various transparent ceramic sintered bodies (terbium-containing composite oxide sintered bodies) which are the objects of the present invention contain the composite oxides shown above as the main component. Here, "contained as a main component" means that any one of the composite oxides is contained at 90% by weight or more. The content of any one of the complex oxides is preferably 99% by weight or more, more preferably 99.9% by weight or more, still more preferably 99.99% by weight or more, and particularly preferably 99.999% by weight or more.

Further, it is preferable to appropriately add a metal oxide functioning as a sintering aid as an auxiliary component. Typical sintering aids include SiO ₂ 、ZrO ₂ 、HfO ₂ CaO, baO, liF, mgO, etc., and carbon (C) other than metal oxide, etc., although this depends on the kind of the material. These components are preferably added in an amount of 0 to 0.5% by weight. It is preferable to add these sintering aids to the terbium-containing oxide sintered body as the main component, because densification can be promoted, residual bubbles can be reduced, and deposition of a heterogeneous phase can be suppressed.

Each of the transparent ceramic sintered bodies (terbium-containing composite oxide sintered bodies) which are the objects of the present invention is composed of the above-described main component and auxiliary component, but may further contain other elements. Examples of the other elements may generally include sodium (Na), phosphorus (P), tungsten (W), tantalum (Ta), and molybdenum (Mo).

The content of the other elements is preferably 10 parts by weight or less, more preferably 0.1 parts by weight or less and particularly preferably 0.001 parts by weight or less (substantially zero) when the total amount of Tb is regarded as 100 parts by weight.

Here, as the ceramic powder for producing the sintered body of the terbium-containing composite oxide, a metal powder, or an aqueous solution of nitric acid, sulfuric acid, uric acid, or the like, of these element groups or oxide powders, or the like, of a series of the above elements (in which terbium is contained and all the elements constituting the respective composite oxides are mixed), can be suitably used.

The purity of the powder is preferably 99.9 wt% or more.

These elements are weighed in predetermined amounts, mixed together, and fired to obtain a fired raw material containing a terbium-containing oxide having a desired composition, specifically, oxides of an aluminum-based garnet type (the above (i)), a gallium-based garnet type (the above (ii)), and a bixbyite type (the above (iii)) forming a solid solution with an element other than terbium as main components.

At this time, the firing temperature needs to be finely adjusted depending on the composition, and cannot be mentioned unconditionally, but is preferably at least 900 ℃ or more and lower than the temperature of sintering to be subsequently performed, more preferably 1000 ℃ or more and lower than the temperature of sintering to be subsequently performed. Incidentally, depending on the raw material, when heating is performed at a certain temperature or higher, adhesion and aggregation may rapidly deteriorate. In the case of using such raw materials, it is preferable to carefully adjust the upper limit temperature and perform firing in a temperature range in which adhesion and aggregation do not deteriorate. Further, such care is not required with respect to the temperature rising rate and the temperature falling rate at the time of firing, but attention is required to the residence time. When the firing residence time is unnecessarily prolonged, the adhesion and aggregation gradually proceed. Therefore, the upper limit range of the residence time also needs to be selected with some degree of care.

Incidentally, "contained as a main component" herein means that a main peak of a powder X-ray diffraction result obtained from a fired raw material is composed of a diffraction peak derived from a crystal system of a desired material. Incidentally, in the case where the presence concentration of the hetero phase is less than 1%, the pattern derived from the main phase can be detected substantially only clearly in the powder X-ray diffraction pattern, and the pattern derived from the hetero phase is often masked almost at a background level.

Subsequently, the obtained fired raw material is pulverized or classified to obtain a ceramic powder in which the particle size distribution is controlled within a predetermined range. The particle size of the ceramic powder is not particularly limited, but it is preferable to select a powder in which the surface of the primary particles has as few facet surfaces as possible because sinterability is improved. Further, it is preferable to subject the purchased raw material to a pulverization treatment such as wet ball mill pulverization, wet bead mill pulverization, wet jet mill pulverization and dry jet mill pulverization, instead of using the raw material as it is, because the generation of coarse particles and coarse pores can be suppressed, thereby producing a dense formed body. Further, as for the purity, it is preferable to select a high-purity starting material powder having a purity of 3N or more because densification in the sintering step is promoted and deterioration of various properties due to impurities can be prevented.

Incidentally, the powder shape at this time is not particularly limited, and for example, square, spherical and plate-like powders can be suitably employed. Further, even a powder subjected to secondary aggregation may be suitably used, and even a granulated powder granulated by a granulation process (such as a spray drying process) may be suitably used.

Further, the step of preparing these ceramic powders is not particularly limited. Ceramic powders prepared by a coprecipitation method, a pulverization method, a spray pyrolysis method, a sol-gel method, an alkoxide hydrolysis method, and any other synthesis method may be suitably used. Further, the obtained ceramic powder may be appropriately treated using a wet ball mill, a bead mill, a wet jet mill, a dry jet mill, a hammer mill, or the like.

In addition, various dispersants may be added during the wet pulverization treatment in order to prevent excessive aggregation of the primary particles. Further, in the case where the primary particles are amorphous and too fluffy or in the case where a starting material having a large aspect ratio and large volume (bulk) such as a plate-like and needle-like material is used, the shape of the primary particles may be set by further adding a calcination step after the pulverization treatment.

In order to manufacture a ceramic sintered body having a composite composition, there are cases where a plurality of ceramic powders are mixed together and molded. In the present invention, such mixed ceramic powder may be used. However, in order to thoroughly mix the ceramic powder before firing, it is preferable to prepare a wet slurry in which the ceramic powder is dispersed in a solvent and subject the wet slurry to a blending treatment such as wet ball mill mixing, wet bead mill mixing, and wet jet mill emulsification. In addition, the various starting materials after mixing may be calcined to cause a phase change to the desired compound.

Further, another example of the transparent ceramic sintered body may include spinel (MgAl) ₂ O ₄ ) And (3) sintering the body. Specifically, the spinel sintered body is a highly transparent spinel sintered body which is useful for an ultraviolet window material, a high-strength window material in a visible light region, an infrared window material, and the like, and several sintering aids are often added to the spinel sintered body in order to improve transparency. However, lithium fluoride, which is added as the simplest method for improving sinterability, cannot be used, since there is thus absorption in the ultraviolet region, but is preferably MgAl ₂ O ₄ Magnesium oxide is added in an amount of 0.08 to 1 wt%, more preferably about 0.1 wt%. Further, since densification is promoted, it is preferable that the additive be uniformly and finely dispersed in the base material.

Further, another example of the transparent ceramic sintered body may include a calcium fluoride/lithium fluoride sintered body. Specifically, another transparent ceramic sintered body is a highly transparent calcium fluoride sintered body which can be used for an optical lens and the like, and in order to improve transparency, lithium fluoride is added to the highly transparent calcium fluoride sintered body at preferably 0.08 wt% or more and less than 3 wt%, more preferably 0.08 wt% or more and 1 wt% or less, particularly preferably about 0.1 wt% of the weight of calcium. However, it is required that lithium fluoride is uniformly and finely dispersed in calcium fluoride as a base material. Further, care is required because even if blending treatment is performed, it is difficult to perform uniform dispersion and mixing when lithium fluoride is added at 3% by weight or more.

Further, examples of the silicon nitride based ceramic sintered body may include a sintered body obtained by blending an oxide based auxiliary (magnesia powder and yttria powder) with silicon nitride powder. Specifically, the silicon nitride-based ceramic sintered body is a silicon nitride ceramic sintered body which has high thermal conductivity and can be used for a heat-radiating substrate, and in order to improve the thermal conductivity, magnesium oxide powder is added to the silicon nitride ceramic sintered body in an amount of preferably 0.01% by weight or more and less than 1% by weight, more preferably 0.05% by weight or more and 0.8% by weight or less, based on the weight of silicon. In addition, in order to improve the insulator pressure and bending strength, yttria powder is added to the silicon nitride ceramic sintered body at preferably 0.01 wt% or more and less than 1 wt%, more preferably 0.05 wt% or more and 0.8 wt% or less, based on the weight of silicon. These oxide additives are preferably uniformly and finely dispersed in the base material because of promoting densification.

Preferably, these various types of ceramic powders are mixed together and subjected to preparation of a raw material slurry. In the present invention, such mixed ceramic powder may be used. However, in order to thoroughly mix the ceramic powder, it is preferable to prepare a wet slurry in which the ceramic powder is dispersed in a solvent for the raw material powder slurry, and to subject the wet slurry to a blending treatment such as wet ball mill mixing, wet bead mill mixing, and wet jet mill emulsification.

The binder added to the raw material slip is a thermoplastic resin having a glass transition temperature higher than room temperature, preferably 3 ℃ or more higher than room temperature. The type of the thermoplastic resin is not particularly limited, but the thermoplastic resin is preferably selected from the group consisting of a conventional polyvinyl alcohol (glass transition temperature Tg =55 ℃ to 85 ℃; depending on the degree of saponification and the degree of polymerization), a polyvinyl acetate (glass transition temperature Tg =25 ℃ to 40 ℃; depending on the degree of saponification and the degree of polymerization), a copolymer of a polyvinyl alcohol and a polyvinyl acetate (glass transition temperature Tg =30 ℃ to 80 ℃; depending on the degree of saponification and the degree of polymerization), a methylcellulose (glass transition temperature Tg = -90 ℃ to 120 ℃; depending on the degree of hydration and the degree of substitution. In the present invention, the glass transition temperature is adjusted to be higher than room temperature), an ethylcellulose (glass transition temperature Tg =70 ℃ to 160 ℃; depending on the degree of substitution), a polyvinyl butyral (glass transition temperature Tg =60 ℃ to 110 ℃; depending on the degree of saponification and the degree of polymerization), a polyvinyl acrylate (glass transition temperature Tg =10 ℃ to 45 ℃; in the present invention, the glass transition temperature is adjusted to be higher than room temperature), and a copolymer of a polyvinyl alcohol and a polyvinyl acrylate (glass transition temperature =15 ℃ to be adjusted to be higher than room temperature; and the degree of saponification). These are preferable because they each suitably have a viscosity, a glass transition temperature thereof is higher than room temperature (or adjusted to be higher than room temperature, preferably adjusted to be higher than room temperature by 3 ℃ or more), and in a range lower than a boiling point of a pressing medium (water or oil) in WIP molding to be described later (or adjusted to be lower than the boiling point, preferably adjusted to be lower than the boiling point by 5 ℃ or more), thereby easily handling the thermoplastic resins. Specifically, the glass transition temperature Tg of the thermoplastic resin is preferably 35 ℃ to 100 ℃, more preferably 40 ℃ to 90 ℃, still more preferably 45 ℃ to 85 ℃.

Incidentally, when the thermoplastic resin is measured by Differential Scanning Calorimetry (DSC), the glass transition temperature Tg is generally a midpoint glass transition temperature value. For example, the glass transition temperature Tg is a midpoint glass transition temperature calculated from a change in heat measured under conditions of a temperature rise rate of 10 ℃ per minute and a measurement temperature of-50 ℃ to 250 ℃ by a method in accordance with JIS K7121: 1987. Incidentally, in the case where the moisture in the sample affects the glass transition temperature Tg, the measurement may be performed after the sample is once heated to 150 ℃ and thus dried.

For adding the thermoplastic resin, it is preferable to use one (thermoplastic resin solution) in which the thermoplastic resin is dissolved in a solvent such as an ethanol solvent in advance so as to have an appropriate concentration (% by weight) in advance, or one (thermoplastic resin dispersion) in which a thermoplastic resin powder whose concentration (% by weight) is adjusted to an appropriate value is dispersed in a solvent such as ethanol even if the thermoplastic powder is not dissolved but separated. In the thermoplastic resin solution or dispersion, the concentration of the thermoplastic resin is preferably, for example, 5 to 40% by weight.

The amount of the thermoplastic resin to be added varies depending on the composition and end use of the intended ceramic sintered body, and therefore an optimum ratio needs to be determined through preliminary experiments. In the present invention, the casting molding is performed using the raw material slip prepared by adding the ceramic powder and the thermoplastic resin as raw material powder components to the solvent, and therefore it is preferable to improve the shape retention at the time of casting molding by increasing the thermoplastic resin addition amount as compared with the case where the particles obtained by spray-drying the raw material slip are subjected to uniaxial press molding or directly subjected to CIP molding. For example, in preparing the raw material slurry, a high-quality molded body and sintered body are obtained when the thermoplastic resin is added in an amount of 2 wt% or more and 40 wt% or less and preferably 3.5 wt% or more and 20 wt% or less of the total weight of the ceramic powder and the thermoplastic resin.

In other words, as a raw material powder component contained in the raw material powder slurry, the ceramic powder is contained at 60 wt% or more and 98 wt% or less and the thermoplastic resin (binder) is contained at 2 wt% or more and 40 wt% or less, preferably, the ceramic powder is contained at 80 wt% or more and 96.5 wt% or less and the thermoplastic resin (binder) is contained at 3.5 wt% or more and 20 wt% or less. At this time, the sum of the ceramic powder and the thermoplastic resin is 100 wt%.

In the present invention, a ceramic powder is dispersed in a solvent to form a wet slurry, and a thermoplastic resin is further added to the wet slurry to form a raw material powder slurry.

The solvent selected for preparing the slurry of ceramic powder is also not particularly limited in the present invention. However, in general, one selected from water, ethanol or an organic solvent (alcohols other than ethanol, acetone, etc.) or a mixture of two or more thereof is suitably selected, and ethanol is particularly preferred. Incidentally, in the case where water is selected as the solvent, it is preferable to simultaneously mix and add a dispersant, an antifoaming agent, and the like. It is necessary to find the optimum range of the respective amounts of these components to be added at this time by preliminary experiments.

Further, a solvent in which the thermoplastic resin added as the binder is dissolved or a solvent in which the thermoplastic resin is insoluble may be used, but a solvent in which the thermoplastic resin is dissolved is preferably used.

In preparing the raw material powder slurry, after adding the thermoplastic resin solution or the thermoplastic resin dispersion to the wet slurry in which the ceramic powder is dispersed in the solvent, the mixture is preferably further thoroughly stirred by performing ball mill mixing, bead mill mixing, wet jet mill mixing, or the like. However, in the case where the calcination treatment is performed in order to change the shape and crystallinity and the average primary particle diameter of the ceramic powder, it is necessary to disperse the ceramic powder in a solvent after the calcination treatment to form a wet slurry, and further to add a thermoplastic resin solution or a thermoplastic resin dispersion to the wet slurry to prevent thermal decomposition, thermal denaturation and thermal volatilization.

Incidentally, in order to improve quality stability and yield in the subsequent ceramic manufacturing step, various organic additives (not including a thermoplastic resin as a binder) may be added to the raw material slurry used in the present invention. In the present invention, these are also not particularly limited. In other words, various dispersants, lubricants, plasticizers, and the like can be suitably used. However, as these organic additives, it is preferable to select a high purity type which does not contain unnecessary metal ions. In addition, the added amount needs to be noted because some types of dispersants have the effect of lowering the glass transition temperature of the thermoplastic resin.

Shaping step

Subsequently, a procedure of molding the ceramic compact for sintering in the present invention is described.

Casting molding

First, a cast molding in which the raw material powder component is wet cast into a predetermined shape is obtained using the thus prepared raw material slip.

Here, the cast molding body is preferably molded by solid-liquid separation by centrifugal casting molding of a slurry (raw material slip) prepared by adding ceramic powder and thermoplastic resin as raw material powder components to a solvent. In more detail, the cast-molded body is more preferably a solid member having a predetermined shape, which is obtained by settling the raw material slurry, filling the obtained aggregate sediment in a container of a centrifugal tube, and subjecting it to a centrifugal separation process (centrifugal casting molding).

A centrifugal casting molding machine used for such centrifugal casting molding has the same configuration as a centrifugal separator, and in which a raw material slurry is filled in a container as a mold, the container is rotated at a high speed, and the raw material slurry is separated into a solid and a liquid by a centrifugal force and molded. In this case, the centrifugal force in the centrifugal separation treatment is preferably 1,000g or more, more preferably 2,000g or more. Further, the time of the centrifugal separation treatment is preferably 20 to 240 minutes, more preferably 30 to 180 minutes.

Subsequently, the obtained cast molding is preferably dried at room temperature to 110 ℃ for 1 to 4 days, more preferably at 40 ℃ to 75 ℃ for 2 to 4 days, whereby a dried cast molding containing the raw material powder component is obtained.

The shape of the dried cast molding corresponds to the desired shape of the sintered body and is, for example, a cylindrical shape having a diameter of 10 to 60 mm and a length of 5 to 40 mm. Alternatively, the shape is a cubic shape having a width of 10 to 85 mm, a thickness of 2 to 30 mm, and a length of 10 to 130 mm.

First stage isostatic compaction

Next, as a first-stage isostatic pressing, the thus-obtained dried cast-molded body is directly inserted into a rubber mold or vacuum-packed and sealed with a waterproof film, and is subjected to isostatic pressing at a temperature lower than the glass transition temperature of the thermoplastic resin contained in the raw material powder component, thereby molding a first-stage press-molded body.

Here, the first-stage isostatic pressing is preferably Cold Isostatic Pressing (CIP) forming. In other words, it is preferable to install the dried cast-molded body directly inserted into a rubber mold or vacuum packed and sealed with a waterproof film on a CIP apparatus and perform first-stage isostatic pressing. The pressing medium in this case is water or oil.

The applied pressure and pressure holding time will now vary depending on the selected ceramic composition and the intended use of the final product and will therefore need to be adjusted appropriately. However, when the applied pressure is not generally increased to 40MPa or more, it is difficult to obtain a high-quality sintered body because the density of the molded body is not increased. The upper limit of the applied pressure is not particularly limited, but it is not preferable to increase the pressure too high because lamination cracking may occur. In most ceramic materials, an applied pressure of 400MPa or less is generally sufficient.

Further, the pressure holding time is, for example, preferably 1 to 10 minutes, more preferably 1 to 3 minutes.

Further, the temperature of the molded body at the time of pressing is kept at a temperature lower than the glass transition temperature of the thermoplastic resin contained in the raw material powder component, and for example, the temperature is preferably kept at a temperature 10 ℃ or more lower than the glass transition temperature of the thermoplastic resin contained in the raw material powder component, and it is particularly preferable to keep the molded body at room temperature without heating.

Incidentally, in this CIP process, it is preferable to keep the temperature at a temperature lower than the glass transition temperature of the thermoplastic resin contained in the raw material powder component at the time of pressing because the thermoplastic resin is firmly solidified in the raw material powder in a state where the gaps between the ceramic powders (primary particles) are completely (densely) filled in advance, and when the raw material powder is press-molded, the pressure applied to the surface of the molded body is transmitted between the hard thermoplastic resin and the hard ceramic powders (primary particles) adjacent to each other in order, and as a result, the pressure is reliably applied to the inside of the molded body.

Second stage isostatic pressing

Subsequently, the obtained first-stage press-formed body is subjected to Warm Isostatic Pressing (WIP) as second-stage isostatic pressing with the body thereof heated to a temperature equal to or higher than the glass transition temperature of the thermoplastic resin, thereby forming a ceramic formed body.

The second stage isostatic pressing is preferably performed by the following procedure.

Step S1

First, in the WIP apparatus for WIP forming, the temperature of the pressurized container portion and the pressing medium for WIP forming is raised in advance to be equal to or higher than the glass transition temperature of the thermoplastic resin contained in the raw material powder component and stabilized.

Step S2

In such a WIP apparatus in a heated state, the first-stage press-formed body is loaded in a state of being filled in a rubber mold or vacuum-packed and sealed with a waterproof film.

Step S3

The first-stage compact is retained while being maintained in the filled state, so that the first-stage compact is heated to the same temperature as that of the heated WIP apparatus, followed by WIP forming as second-stage, etc. static press forming.

Incidentally, in step S3, after the first-stage press-formed body is filled, WIP forming may be performed as second-stage, etc. static press-forming while heating the first-stage press-formed body to the same temperature as the WIP apparatus.

Here, the temperature of the pressurized container portion and the pressing medium for WIP molding, that is, the temperature at which the first-stage press-molded body is heated, is equal to or higher than the glass transition temperature of the thermoplastic resin contained in the raw material powder component, and is preferably 5 ℃ or higher than the glass transition temperature of the thermoplastic resin. Incidentally, in the case where the glass transition temperature of the thermoplastic resin is 50 ℃ or less, it is preferable to set the temperature at which the first-stage press-molded body is heated to a temperature higher than the glass transition temperature by 10 ℃ or more. Further, the upper limit of the temperature for heating the first-stage press-formed body is preferably 130 ℃ or lower.

Furthermore, the pressing medium to be used is preferably water or oil, more preferably water or oil having a boiling point of more than 100 ℃. At this time, although the pressing medium differs depending on the kind of the selected thermoplastic resin, the use of water as the pressing medium is dangerous because there is a possibility of bumping in the case where the glass transition temperature of the thermoplastic resin is 91 ℃ or more. Therefore, it is preferred to select oils with boiling points above 100 ℃ as the pressing medium. Incidentally, there are various oils, and therefore it is preferable to appropriately select an oil that does not have a risk of bumping even when heated to a desired temperature.

The pressure applied and the pressure holding time in WIP molding vary depending on the selected ceramic composition, the kind and addition ratio of the thermoplastic resin, and the intended use of the final product, and thus need to be appropriately adjusted. However, unless the applied pressure is generally increased to 40MPa or more, the thermoplastic resin heated to a temperature higher than the glass transition temperature is difficult to pass between the gaps of the stacked ceramic molded bodies and cause plastic flow, whereby rearrangement of the raw materials inside the molded body in a compressed state and storage internal stress do not occur in the CIP step, and it is difficult to reduce residual stress, close coarse cavities, and further densify the molded body. The upper limit of the applied pressure is not particularly limited, but it is generally known that the maximum ultimate pressure of the WIP equipment is lower than the maximum ultimate pressure of the CIP equipment. This is a limitation in the production equipment for safe operation of the equipment, including thermal expansion of the entire equipment due to elevated temperatures. Specifically, the maximum applied pressure of the WIP apparatus is generally about 200MPa, but in the present invention, if such a degree of pressure is applied, it can sufficiently function.

Further, the pressure holding time is, for example, preferably 1 to 10 minutes and more preferably 1 to 3 minutes.

Incidentally, the first-stage isostatic pressing and the second-stage isostatic pressing may be performed as follows, instead of as in the embodiments thereof described above.

First stage isostatic compaction

The dry cast molding is inserted into a rubber mold or vacuum-packed and sealed with a waterproof film, mounted on a WIP apparatus, and molded under first-stage isostatic pressing conditions (first-stage press molding is produced).

Second stage isostatic pressing

After the first stage press formed body is formed, heating of the first stage press formed body is started while maintaining the first stage isostatic pressing state (that is, the first stage press formed body is mounted on the WIP apparatus and pressed), and then WIP forming is performed under the conditions of the second stage isostatic pressing.

In the present invention, it is essential that the first-stage press forming (isostatic press forming at a temperature lower than the glass transition temperature of the thermoplastic resin, preferably CIP forming) step and the second-stage isostatic press forming (WIP forming) step are performed in this order.

For example, in the case of observing the distribution state of the ceramic powder and the binder (thermoplastic resin) in the dry cast molding body after the cast molding and drying, the ceramic powder and the binder (thermoplastic resin) are in a state in which the ceramic powder and the binder (thermoplastic resin) are relatively uniformly dispersed, or a state in which the binder (thermoplastic resin) is present to fill the gaps between the ceramic powders, but the density of the molding body is in a relatively low state.

Next, the first-stage press molding (isostatic press molding at a temperature lower than the glass transition temperature of the thermoplastic resin, preferably CIP molding) in the first half of isostatic press molding has a function of transmitting an applied pressure to the inside of the thick molded body with respect to the dried cast molded body. At this time, in the first-stage compact, although a state in which the ceramic powder and the binder (thermoplastic resin) are relatively uniformly dispersed in the dried cast compact or a state in which the binder (thermoplastic resin) is present to bury the gaps between the ceramic powders is maintained, the density of the first compact becomes larger than that of the dry cast compact to some extent.

Then, the second-stage isostatic pressing (WIP forming) step in the latter half of isostatic pressing has a function of eliminating the negative effects caused by the first-stage press forming step, that is, the internal stress deformation of the formed body and the generation of partially coarse voids by plastic flow and rearrangement in the first-stage press formed body. At this time, in the ceramic compact, although a state in which the ceramic powder and the binder (thermoplastic resin) are relatively uniformly dispersed in the first-stage press-formed body or a state in which the binder (thermoplastic resin) is present to fill the gaps between the ceramic powders is maintained, the density of the ceramic compact is larger than that of the first-stage press-formed body.

The range of conditions set differently in these two isostatic pressing steps is arbitrary as long as the series of actions of these first and second stages of isostatic pressing functions properly.

However, there is a need to verify and verify that the series of actions is functioning properly. This confirmation and verification is preferably performed by the following method.

In other words, the first confirmation method is to confirm the density d of the molded body _CIP+WIP >Density d of the shaped body _CIP Because the density d of the formed body (ceramic formed body) in which a series of actions of the first and second stages of isostatic pressing after the first stage press-forming step and the second stage press-forming step correctly acts _CIP+WIP Necessarily greater than the density d of the formed body after the first-stage press forming step _CIP 。

Furthermore, as a second confirmation method, it was confirmed that the density d of the molded body was _WIP <Density d of the shaped body _CIP+WIP Because when a sample in which the dried cast molding body was not subjected to the first-stage press molding but only the second-stage press molding (WIP molding) was produced for comparison, the density d of the molding body subjected to only the WIP molding was _WIP Lower than the density d of the shaped body _CIP+WIP . Incidentally, coarse voids are formed inside the formed body to which the first-stage press forming is not applied in this manner but only the second-stage press forming (WIP forming) is applied.

As described above, according to the method of producing a ceramic molded body for sintering of the present invention, when a ceramic molded body (particularly, a thick ceramic molded body) is press-molded, pressure transmission to the inside of the molded body and plastic flow of a thermoplastic resin can be effectively achieved, and a dense ceramic molded body in which residual voids are significantly small and residual stress is eliminated can be obtained.

Method for producing ceramic sintered body (densification of ceramic)

The method of producing a ceramic sintered body of the present invention is to obtain a ceramic sintered body by sintering the ceramic compact produced by the method of producing a ceramic compact for sintering of the present invention, and further Hot Isostatic Pressing (HIP) the sintered ceramic compact for further densification.

At this time, the method preferably further comprises a step of degreasing the ceramic compact before sintering. In addition, the method preferably further comprises a step of annealing the HIP-treated body after the HIP treatment.

Specifically, the following processing is performed.

Degreasing

In the production method of the present invention, a conventional degreasing step may be suitably employed. In other words, the temperature-programmed degreasing step may be performed using a general heating furnace. Further, the kind of the atmosphere gas at this time is not particularly limited, and air, oxygen gas, oxygen-containing mixed gas, hydrogen gas, fluorine gas, hydrofluoric acid gas, nitrogen gas, ammonia gas, and the like can be suitably used. The degreasing temperature is also not particularly limited, but in the case where a thermoplastic resin is added as well as a dispersant and other organic substances, it is preferable that the temperature is raised to and maintained at a temperature at which all organic components thereof can be completely decomposed and eliminated.

Sintering

In the production method of the present invention, a general sintering step can be suitably employed. In other words, a heating and sintering step by a resistance heating method or an induction heating method may be suitably employed. The atmosphere at this time is not particularly limited, and sintering can be performed in various types of atmospheres such as inert gas, oxygen gas, hydrogen gas, fluorine gas, hydrofluoric acid gas, argon gas, nitrogen gas, and ammonia gas, or under reduced pressure (in vacuum). However, since the favorable compatible gas varies depending on the kind of the ceramic material to be processed, it is preferable to properly cope with this condition. For example, the atmosphere is preferably selected from an oxygen-based gas group or a reduced pressure atmosphere in the case of oxide ceramics, from a fluorine gas, a hydrofluoric acid-based gas group, an inert gas such as argon and nitrogen, or a reduced pressure atmosphere in the case of fluoride ceramics, and from a nitrogen gas, an amino gas group, or a reduced pressure atmosphere in the case of nitride ceramics. In addition, it is needless to say that the material selection and the gas-tightness management of the furnace should be performed according to the kind of gas used.

The sintering temperature in the sintering step of the present invention needs to be appropriately adjusted according to the selected composition and crystal system. In general, it is preferable to perform the sintering process in a temperature region several tens to several hundreds degrees below the melting point of the ceramic material having the intended final composition. Furthermore, a sintering residence time of several hours in the sintering step is generally sufficient. However, unless a porous sintered body is intentionally produced, the relative density of the sintered body needs to be increased to at least 95% or more. Further, it is more preferable to increase the relative density of the sintered body to 99% or more by performing the sintering treatment for a long time of 10 hours or more, because the final transparency of the transparent ceramic sintered body is further improved.

Incidentally, it is important to select the temperature rise rate in the sintering step. It is preferable to select the temperature rise rate as small as possible, but there is also a limit due to productivity and cost limitations. Therefore, it is preferable that a minimum of 100 ℃/hr can be secured. The temperature increase rate may preferably be set to a small degree because densification can be promoted, transparency can be improved, and segregation and cracking can be suppressed.

Hot Isostatic Pressing (HIP)

In the manufacturing method of the present invention, a Hot Isostatic Pressing (HIP) treatment is additionally performed after the sintering step.

Incidentally, as the kind of the pressing gas medium at this time, argon gas, inert gas such as nitrogen gas, or Ar-O may be suitably used ₂ . The pressure applied by the pressing gas medium is preferably 50 to 300MPa, more preferably 100 to 300MPa. When the pressure is less than 50MPa, the effect of improving the densification may not be obtained, and the densification may not be further improved, and even when the pressure is increased to exceed 300MPa, the load on the equipment becomes excessive, and the equipment may be damaged. It is simple and preferred that the applied pressure is below 196MPa, since the treatment can be carried out by commercially available HIP equipment at this pressure.

Incidentally, in the case where the sintered body selected is a fluoride, it is preferable to perform a so-called capsule HIP treatment in which the HIP treatment is performed after the sintered body is sealed with a low-carbon steel capsule or the like.

Further, the temperature (predetermined holding temperature) at the time of the HIP treatment is set in the range of 1,000 ℃ to 1,800 ℃ and preferably 1,100 ℃ to 1,700 ℃. A heat treatment temperature exceeding 1,800 deg.c is not preferable because the risk of permeation of a medium gas into the sintered body or melting and sticking of the sintered body and the HIP furnace to each other increases. Further, when the heat treatment temperature is lower than 1,000 ℃, the effect of improving the densification of the sintered body is hardly obtained. Incidentally, the residence time at this heat treatment temperature is not particularly limited, but too long residence time is not preferable because defects in the sintered body gradually accumulate. Generally, the residence time is suitably set to 1 to 3 hours.

Incidentally, the heater material, the heat insulating material, and the processing container used in the HIP treatment are not particularly limited, but graphite or molybdenum (Mo), tungsten (W), and platinum (Pt) may be suitably employed, and furthermore, yttrium oxide, gadolinium oxide, silicon carbide, and tantalum carbide may also be suitably used as the processing container. Incidentally, it is preferable that the sintered body be a group of sintered bodies under relatively low temperature conditions in which the HIP treatment temperature is 1,500 ℃ or less, because platinum (Pt) can be used as a heater material, a heat insulating material and a treatment vessel, the degree of freedom in selection of the atmosphere to be selected increases, and the concentration of point defects in the obtained sintered body can be reduced. Further, in the case where the treatment temperature is 1,500 ℃ or more, graphite is preferable as the heater material and the heat insulating material.

Annealing of

In the production method of the present invention, there are cases where: in the production of a transparent ceramic sintered body, point defects are generated in the transparent ceramic sintered body obtained after termination of the HIP treatment, the transparent ceramic sintered body having a light gray or black gray appearance. In such cases, it is preferable to perform the annealing treatment (defect recovery treatment) at a temperature equal to or lower than the HIP treatment temperature (typically 1,000 ℃ to 1,500 ℃), in an oxygen atmosphere in the case of an oxide, in a fluorine or hydrofluoric acid atmosphere in the case of a fluoride, and in a nitrogen or ammonia atmosphere in the case of a nitride. The residence time in this case is preferably 3 hours or more because it is necessary to secure a sufficient time to recover the point defect. Incidentally, it is not preferable to increase the set temperature in the annealing treatment step to more than 1,500 ℃ or excessively extend the residence time to several tens of hours, because bubbles are regenerated everywhere in the transparent ceramic material (this is called a rebound phenomenon).

Optical evaluation

In the manufacturing method of the present invention, in the case of manufacturing a transparent ceramic sintered body, in order to evaluate the optical quality of the sintered body that has been subjected to a series of the above-described manufacturing steps, it is preferable to optically polish at least one surface. The optical surface precision is not particularly limited at this time. However, when the warpage of the optical surface is too severe, it is difficult to perform correct optical evaluation, and therefore the optical surface accuracy is preferably λ or less, more preferably λ/2 or less, and particularly preferably λ/4 or less, for example, in the case where the measurement wavelength λ =633 nm. Incidentally, by appropriately forming the antireflection film on the optically polished surface, the optical loss can be further reduced.

By microscopic observation of the inside through the optically polished surface in the above manner, the presence or absence of residual bubbles, coarse cavities, and residual distortion can be observed and evaluated by cross Nicol images or the like.

As described above, according to the method of manufacturing a ceramic sintered body of the present invention, a ceramic sintered body having a truly high density and significantly less residual bubbles can be manufactured, and as a result, a high-quality ceramic sintered body exhibiting improved mechanical strength, thermal conductivity, optical transparency, electrical characteristics, long-term reliability, and the like, and thus exhibiting more advantageous properties compared to the prior art can be obtained.

Examples

Hereinafter, the present invention is described more specifically with reference to examples and comparative examples, but the present invention is not limited to the examples.

Example 1

Terbium oxide powder and scandium oxide powder manufactured by Shin-Etsu Chemical co. Further, a liquid of tetraethyl orthosilicate (TEOS) manufactured by KISHIDA CHEMICAL co. The purity of the powder raw material is 99.9 wt% or more, and the purity of the liquid raw material is 99.999 wt% or more.

Using this raw material and adjusting the mixing ratio thereof, a raw material of garnet-type oxide having the final composition shown in table 1 was prepared (mixed raw material No. 1).

In other words, a mixed powder in which the mole numbers of terbium and aluminum and scandium were each in the mole ratio in the composition shown in table 1 was prepared by weighing the raw materials. Subsequently, TEOS was weighed and added to the raw material so that the amount added was SiO ₂ In the weight percentage (0.01 wt%) shown in table 1. Subsequently, the mixed powder was dispersed and mixed in ethanol using an alumina ball milling apparatus. The treatment time was 15 hours.

Incidentally, the mixed raw material No.1 alone was further subjected to a spray drying treatment to produce a granular raw material having an average particle diameter of 20 μm. Subsequently, the granular raw material was put into a yttria crucible and fired at 1,200 ℃ for a residence time of 3 hours using a high-temperature muffle furnace, thereby obtaining a fired raw material. The diffraction pattern of the obtained fired raw material was analyzed using a powder X-ray diffractometer manufactured by Malvern Panalytical ltd. (XRD analysis), and as a result, it was confirmed that the fired raw material was composed of only garnet single phase (cubic) by comparison of the actually measured pattern with reference data of the X-ray diffraction pattern.

TABLE 1

The mixed raw material No.1 was dispersed and mixed again in ethanol using a nylon ball mill apparatus. The treatment time was 20 hours. The wet slurries thus obtained were divided into two groups, and a thermoplastic resin solution in which a copolymer of polyvinyl alcohol and polyvinyl acetate (glass transition temperature: 48 ℃) produced by JAPAN VAM & POVAL co., ltd., as a binder was dissolved in ethanol to 20 wt% was weighed and added to one group so that the copolymer of polyvinyl alcohol and polyvinyl acetate was present in an amount of 4 wt% of the weight of the entire raw material powder (mixed raw material No.1+ binder), and then the slurry to which the binder was added (raw material powder slurry (1)) was stirred and mixed for 3 hours. The same thermoplastic resin solution was weighed and added to the other group so that the copolymer of polyvinyl alcohol and polyvinyl acetate was present in an amount of 1% by weight based on the weight of the whole raw material powder (mixed raw material No.1+ binder), and then the binder-added slurry (raw material powder slurry (2)) was stirred and mixed for 3 hours. In both the raw material powder slurries (1) and (2), the binder is dissolved in the slurry.

The raw material slurries (1) and (2) divided into these two groups were each left standing for 24 hours to settle, thereby producing aggregated sediments. The supernatants were each removed, the remaining aggregate sediment was packed into 10 mm diameter round-bottom centrifuge tubes and placed in a centrifugal separator, and these round-bottom centrifuge tubes were spun at a maximum centrifugal force of 3,000g for 1 hour (60 minutes) to centrifuge the aggregate sediment packed in the round-bottom centrifuge tubes. After completion of the centrifugation, the supernatant was removed again, and then the solid molded body was collected (centrifugally cast molded body). The collected solid formed body was oven-dried at 60 ℃ for 48 hours, thereby obtaining a dried cast formed body. The above process is called "casting & drying".

The obtained dried cast-molded bodies were further divided into five groups as shown in Table 2 (example 1-1 and comparative examples 1-1 to 1-4). Subsequently, ceramic molded body samples (three levels of casting & drying-CIP process-WIP process, casting & drying-CIP process, casting & drying-WIP process) were produced by the molding procedure under the conditions shown in table 2 (in the table, mark "∘" indicates that the pressing process was performed, mark "-" indicates that the pressing process was not performed (the same applies hereinafter)).

Incidentally, the room temperature in this molding procedure was 20 ℃. CIP conditions were set as follows: pressing a medium: water, pressing medium temperature: 20 ℃, applied pressure: 196MPa, pressing time: for 2 minutes. Further, the WIP conditions are set as follows: pressing a medium: water, pressing medium temperature: heating temperature of CIP molded body at 60 ℃:60 ℃, applied pressure: 196MPa, pressing time: for 2 minutes.

For the obtained ceramic molded body samples, the weight w (g) of each sample was measured, and the diameter r (mm) and the length L (mm) were also measured, and the density of each molded body was determined by the following equation calculation.

Shaping ofBulk Density (g/cm) ³ )＝(4,000w)/(πr ² L)

Further, the appearance of the ceramic molded body sample was visually observed.

Next, each ceramic molded body was degreased in a muffle furnace at 1,000 ℃ for 2 hours. Subsequently, the degreased ceramic formed body was charged into an oxygen atmosphere furnace and sintered at 1,730 ℃ for 3 hours to obtain a sintered body. Further, each sintered body was charged into a HIP furnace made of a carbon heater, and subjected to HIP treatment under an Ar atmosphere under conditions of an applied pressure of 200MPa, a heating temperature of 1,600 ℃, and a residence time of 2 hours. Subsequently, each of the obtained HIP-treated sintered bodies was charged into an oxygen atmosphere furnace and subjected to an annealing treatment at a heating temperature of 1,350 ℃ for a retention time of 4 hours to obtain a ceramic sintered body in which oxygen deficiency was recovered.

Each of the thus obtained ceramic sintered bodies was ground and polished to have a diameter of 5mm and a length of 15mm, and further subjected to final optical polishing on both optical end faces of each of the ceramic sintered bodies to have an optical surface precision λ/2 (the case where the measurement wavelength λ =633 nm), thereby obtaining a sample for evaluation.

Next, the total light transmittance and the forward scattering coefficient were measured for each sample as follows. Here, the n value of each sample is set to 3, and the average value of the measurement results is regarded as the actual measurement value of each sample (the same applies hereinafter).

Method for measuring total light transmission and forward scattering coefficient

Total light transmittance at a wavelength of 1,064nm was measured using a spectrophotometer V-670 manufactured by JASCO Corporation. As a measurement method, first, light spectrally diffracted by the spectrophotometer (light having a wavelength of 1,064nm (the same applies hereinafter)) is irradiated in the spectrophotometer V-670 without providing a sample for evaluation, the light is received by an integrating sphere previously provided in the apparatus, and the converged light is received by a detector. The obtained illuminance is represented by I ₀ The sample for evaluation is then set in the apparatus, whereupon the spectrally diffracted light is incident on the sample, this time for evaluation, and the transmitted light is again converged by the integrating sphere and received by the detector. Obtained illumination representationFor I, the total light transmittance is determined by equation (1).

Furthermore, the forward scattering coefficient is measured continuously. In other words, the setting of the integrating sphere is switched to a mode in which linearly transmitted light is removed, light spectrally diffracted again in a state in which the sample for evaluation is set is incident on the sample for evaluation, and light other than the linearly transmitted light of the transmitted light is condensed by the integrating sphere and received by the detector. The obtained illuminance is represented as I _s And the forward scattering coefficient is determined by equation (2).

Total light transmittance (%/15 mm) = I/I ₀ X 100 equation (1)

Forward scattering coefficient (%/15 mm) = Is/I ₀ X100 equation (2)

The above results are summarized in table 2.

TABLE 2

*1: adhesive: copolymer of polyvinyl alcohol and polyvinyl acetate (Tg: 48 ℃ C.)

*2: CIP conditions: the pressing medium temperature is 20 ℃, the applied pressure is 196MPa, and the pressing time is 2 minutes

*3: WIP conditions: the temperature of the pressing medium is 60 ℃, the applied pressure is 196MPa, and the pressing time is 2 minutes

*4: the addition amount of the raw material powder component in the slurry

From the above results, in example 1-1 in which the molding procedure of casting & drying-CIP process-WIP process was performed using the raw material slip to which the binder (thermoplastic resin) was added at 4 wt%, the density of the molded body was maximally improved, the total light transmittance after sintering was the highest, and the forward scattering coefficient was the lowest. Even in the case of the raw material slurries to which the binders were added in the same manner, in comparative examples 1-1 and 1-2 in which the molding procedures of the casting & drying-CIP treatment or the casting & drying-WIP treatment were performed, the density of the molded body was hardly increased, the total light transmittance was slightly lowered as compared with example 1-1, and the forward scattering coefficient was also deteriorated. Incidentally, it can be confirmed from the comparison between comparative examples 1-1 and 1-2 that the density of the molded body produced under the molding conditions of the cast & dry-WIP treatment is higher than that of the molded body produced under the molding conditions of the cast & dry-CIP treatment, but the molded body is not good in the total light transmittance and the forward scattering coefficient at the time of sintering. Further, in the raw material slurries (comparative examples 1 to 3 and 1 to 4) in which the amount of the binder added was 1% by weight, the compact density was to some extent, but cracks occurred on a part of the ceramic compact, and the total light transmittance and the forward scattering coefficient were not so favorable under the molding conditions of both the casting & drying-CIP process and the casting & drying-CIP process-WIP process. This is presumably because most of the specifically added binder has been discharged from the system during casting and drying, and the net amount of binder remaining in the shaped body has been greatly reduced to below 1% by weight.

Example 2

Spinel (MgAl) manufactured by taiimei CHEMICALS co ₂ O ₄ ) And (3) powder. The purity thereof was 99.99 wt% or more (expressed as 4N-MgAl) ₂ O ₄ ). Further, a magnesium oxide powder manufactured by Ube Material industries, ltd. The purity thereof is 99.99 wt% or more.

The raw materials were weighed so that the magnesia powder was based on 4N-MgAl in terms of MgO ₂ O ₄ Is present in an amount of 0.1 wt% and is mixed together to prepare a starting material, which is subsequently dispersed and mixed in ethanol using an alumina ball milling apparatus. The treatment time was 15 hours. Subsequently, a spray drying treatment was further performed, thereby producing a granular raw material having an average particle diameter of 20 μm.

The granular raw material was put into an alumina crucible and fired at 650 ℃ for a retention time of 3 hours using a muffle furnace, thereby obtaining a fired raw material (fired raw material No. 1). The diffraction pattern of the obtained fired raw material was analyzed using a powder X-ray diffractometer manufactured by Malvern Panalytical ltd. (XRD analysis), and as a result, it was confirmed that the sample consisted of only a spinel single phase by comparison of the measured pattern with reference data of the X-ray diffraction pattern.

The fired raw materials were again dispersed and mixed in ethanol using an alumina ball mill apparatus. The treatment time was 20 hours. The wet slurries thus obtained were divided into two groups, a thermoplastic resin solution in which polyvinyl alcohol (glass transition temperature: 60 ℃) manufactured by JAPAN VAM & POVAL co., ltd. as a binder was dissolved in ethanol to 20 wt% was weighed and added to one group so that the polyvinyl alcohol was present in an amount of 4 wt% of the weight of the entire raw material powder (fired raw material No.1+ binder), and then the slurry to which the binder was added (raw material powder slurry (1)) was stirred and mixed for 3 hours. The same thermoplastic resin solution was weighed and added to the other group so that polyvinyl alcohol was present in an amount of 1 wt% based on the weight of the whole raw material powder (fired raw material No.1+ binder), and then the binder-added slurry (raw material powder slurry (2)) was stirred and mixed for 3 hours. In both the raw material slurries (1) and (2), the binder is dissolved in the slurry.

The raw material slurries (1) and (2) divided into these two groups were each left standing for 24 hours to settle, thereby producing aggregated sediments. The supernatants were each removed, the remaining aggregate sediment was packed into 10 mm diameter round bottom centrifuge tubes and placed in a centrifuge separator, and these round bottom centrifuge tubes were spun at 2,000g maximum centrifugal force for 3 hours (180 minutes) to centrifuge the aggregate sediment packed in the round bottom centrifuge tubes. After completion of the centrifugation, the supernatant was removed again, and then the solid molded body was collected (centrifugally cast molded body). The collected solid formed body was oven-dried at 60 ℃ for 48 hours, thereby obtaining a dried cast formed body. The above process is called "casting & drying".

The obtained dried cast-molded bodies were further divided into five groups as shown in Table 3 (example 2-1 and comparative examples 2-1 to 2-4). Subsequently, ceramic compact samples were manufactured by the molding procedures (three levels of casting & drying-CIP process-WIP process, casting & drying-CIP process, and casting & drying-WIP process) under the conditions shown in table 3.

Incidentally, the room temperature in this molding procedure was 20 ℃. CIP conditions were set as follows: pressing a medium: water, pressing medium temperature: 20 ℃, applied pressure: 196MPa, pressing time: for 2 minutes. Further, the WIP conditions are set as follows: pressing a medium: water, pressing medium temperature: 70 ℃, heating temperature of CIP molded body: 70 ℃, applied pressure: 196MPa, pressing time: for 2 minutes.

For the obtained ceramic shaped bodies, the weight w (g) of each sample was measured, and the diameter r (mm) and the length L (mm) were also measured, and the density of each shaped body was determined by the following equation calculation.

Density (g/cm) of the shaped body ³ )＝(4,000w)/(πr ² L)

Subsequently, each ceramic molded body was degreased in a muffle furnace at 800 ℃ for 3 hours. Subsequently, the degreased ceramic formed body was charged into an oxygen atmosphere furnace and sintered at 1,450 ℃ for 3 hours to obtain a sintered body. Further, each sintered body was charged into a HIP furnace made of a carbon heater, and HIP treatment was applied under conditions of a pressure of 200MPa, a heating temperature of 1,550 ℃, and a residence time of 2 hours in an Ar atmosphere. Subsequently, each of the obtained HIP-treated sintered bodies was charged into an oxygen atmosphere furnace and subjected to an annealing treatment at a heating temperature of 1,100 ℃ for a retention time of 4 hours to obtain a ceramic sintered body in which oxygen deficiency was recovered (spinel sintered body).

For each sample thus obtained, the total light transmittance and the forward scattering coefficient were measured under the same measurement conditions as in example 1. The results obtained are summarized in table 3.

TABLE 3

*1: fired raw material No.1: spinel powder + magnesia powder (0.1 mass% Mg in terms of MgO relative to spinel)

*2: adhesive: polyvinyl alcohol (Tg: 48 ℃ C.)

*3: CIP conditions: the temperature of the pressing medium is 20 ℃, the applied pressure is 196MPa, and the pressing time is 2 minutes

*4: WIP conditions are as follows: the temperature of the pressing medium is 70 ℃, the applied pressure is 196MPa, and the pressing time is 2 minutes

*5: the addition amount of the raw material powder component in the slurry

From the above results, in example 2-1 in which the molding procedure of casting & drying-CIP process-WIP process was performed using the raw material slip to which the binder (thermoplastic resin) was added at 4 wt%, the density of the molded body was maximally improved, the total light transmittance after sintering was the highest, and the forward scattering coefficient was the lowest. Even in the case of the raw material slip to which the binder was added in the same manner, in comparative examples 2-1 and 2-2 in which the molding procedure of the casting & drying-CIP process or the casting & drying-WIP process was performed, the density of the molded body was hardly increased, the total light transmittance was slightly lowered as compared with example 2-1, and the forward scattering coefficient was also deteriorated. Incidentally, it can be confirmed from the comparison between comparative example 2-1 and comparative example 2-2 that the density of the molded body produced under the molding conditions of the casting & drying-WIP treatment is higher than that of the molded body produced under the molding conditions of the casting & drying-CIP treatment, but the molded body is not good in total light transmittance and forward scattering coefficient at the time of sintering. Further, in the raw material slurries (comparative examples 2-3 and 2-4) in which the amount of the binder added was 1 wt%, the ceramic formed bodies were cracked, and the total light transmittance and the forward scattering coefficient could not be measured under the forming conditions of both the casting & drying-CIP process and the casting & drying-CIP process-WIP process. In other words, it is considered that this is because the shape retention of the ceramic formed body is insufficient when the amount of the binder added is 1 wt% as discussed in example 1.

As described above, as shown in the embodiments of the present invention, when a raw material slip is prepared by previously adding a thermoplastic resin at 2 wt% or more and 40 wt% or less as a raw material powder component, a cast molding body is manufactured by wet-casting the raw material slip, drying and then Cold Isostatic Pressing (CIP) molding is applied at room temperature lower than the glass transition temperature of the thermoplastic resin, and then Warm Isostatic Pressing (WIP) molding is applied to the CIP molding body while heating at a temperature equal to or higher than the glass transition temperature of the thermoplastic resin, a dense and favorable ceramic molding body in which residual voids are significantly small and residual stress is eliminated can be manufactured, when the ceramic powder is molded. Further, by performing a sintering process using the ceramic compact, a ceramic sintered body having a truly high density and significantly less residual bubbles can be produced. As a result, a high-quality ceramic sintered body can be provided, which exhibits improved optical transparency, mechanical strength, and thermal conductivity, and thus exhibits more advantageous properties than the prior art.

Incidentally, the present invention has been described using the above-described embodiment, but the present invention is not limited to the embodiment, and the present invention may be modified within the scope of those skilled in the art, such as other embodiments, additions, modifications, and deletions, as long as the effects of the present invention are exerted, and any aspect is within the scope of the present invention.

Japanese patent application Nos. 2018-092085 and 2019-073264 are incorporated herein by reference.

Although preferred embodiments have been described, many modifications and variations are possible in light of the above teaching. It is, therefore, to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

Claims

1. A method for producing a ceramic molded body for sintering, which is molded by isostatic pressing a raw material powder containing a ceramic powder and a thermoplastic resin having a glass transition temperature higher than room temperature, the ceramic powder being an oxide, nitride or fluoride or metal-based material, into a predetermined shape, the method comprising the steps of:

preparing a raw material slip by adding a ceramic powder and a thermoplastic resin to a solvent such that the thermoplastic resin is present in an amount of 2 wt% or more and 40 wt% or less of the total weight of the ceramic powder and the thermoplastic resin;

forming a first-stage press-formed body by isostatic pressing a dried cast-formed body at a temperature lower than the glass transition temperature of the thermoplastic resin as a first-stage isostatic pressing; and

forming a ceramic shaped body by: warm Isostatic Pressing (WIP) the first-stage press-formed body as a second-stage isostatic pressing with the body heated to a temperature equal to or higher than the glass transition temperature of the thermoplastic resin,

wherein the pressure holding time for the first-stage isostatic pressing is 1 to 10 minutes, and the temperature of the molded body at the time of pressing is held at 10 ℃ or more below the glass transition temperature of the thermoplastic resin.

3. The method as set forth in claim 1, wherein after the first-stage press-formed body is formed, heating of the first-stage press-formed body is started while maintaining the first-stage isostatic pressing state, and then WIP forming is performed as the second-stage isostatic pressing.

4. The method of claim 1 wherein the pressing medium in WIP forming is water or oil.

5. The method of claim 1, wherein the thermoplastic resin has a glass transition temperature above room temperature and below the boiling point of the pressing medium in WIP molding.

6. The method of claim 1, wherein the thermoplastic resin is at least one selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, a copolymer of polyvinyl alcohol and polyvinyl acetate, methyl cellulose, ethyl cellulose, polyvinyl butyral, polyvinyl acrylate, and a copolymer of polyvinyl alcohol and polyvinyl acrylate.

7. The method of claim 1, wherein the cast molding body is molded by centrifugal casting molding via solid-liquid separation of a raw material slip.

8. A method of manufacturing a ceramic sintered body, the method comprising the steps of: preparing a ceramic shaped body as claimed in claim 1, sintering the ceramic shaped body in an inert atmosphere or in a vacuum and further Hot Isostatic Pressing (HIP) the sintered ceramic shaped body.

10. The method of claim 8, further comprising the step of annealing the HIP treatment after HIP treatment.